Geoscience Reference
In-Depth Information
800
CH 4 (g)
600
400
CO 2 (g)
200
5
88 kyr
85 kyr
108 kyr
122 kyr
0
0
-5
Temperature
-10
300
225
150
75
0
Thousands of years before present
Figure 12.20. Methane, carbon dioxide, and temperature variations from the Vostok ice core over the past
425,000 to 450,000 years. Temperature variations are relative to a modern surface air temperature over the ice
of
55 C. From Jouzel et al. (1987, 1993, 1996); Petit et al. (1999).
12.3.2.7. Changes in the Eccentricity
of the Earth's Orbit
Figure 12.21 shows that the Earth travels around the
sun in an elliptical pattern, with the sun at one focus.
In the figure, a and b are the lengths of the major and
minor semi-axes, respectively, of the ellipse. The dis-
tance between the center of the ellipse (point C) and
either focus is c ,which is related to a and b by the
Pythagorean relation, a 2
The eccentricity varies between 0 and 1. A circle has an
eccentricity of 0. Earth's eccentricity is currently low,
0.017 (Example 12.2), indicating that the Earth's orbit
is nearly circular but still noncircular enough to create
a3.4 percent difference in Earth-sun distance between
June and December.
Example 12.2
Calculate the current eccentricity of the Earth.
Solution
Figure 12.1 shows that the Earth-sun distance
during the winter solstice (when the Earth is clos-
est to the sun) is currently a
=
b 2
+
c 2 .The eccentricity
( e )ofthe ellipse is
c
a
e
=
(12.7)
147.1million km
and during the summer solstice (when the Earth
is furthest from the sun) is a
c
=
152.1 million
km. Solving these two equations gives c
+
c
=
High-eccentricity orbit
Low-eccentricity orbit
=
2.5
million km and a
149.6 million km. Substituting
these numbers into Equation 12.7 gives the cur-
rent eccentricity of the Earth's orbit around the
sun as e
=
Earth
Earth
a
a
b
b
c
a
c
a
0.017.
C
C
Sun
Sun
The eccentricity of the Earth varies sinusoidally
with a period of roughly 100,000 years. The minimum
and maximum eccentricities during each 100,000-year
period vary as well. The minimum eccentricity is usu-
ally greater than 0.01, and the maximum is usually less
than 0.05. Whereas today the Earth is in an orbit of
Figure 12.21. Earth's orbit around the sun during
periods of high and low orbital eccentricity.The
eccentricities are exaggerated in comparison with
real eccentricities of the Earth's orbit.
Search WWH ::




Custom Search